Dihydroxy-acid dehydratase (DHAD), also called acetohydroxy acid dehydratase, catalyzes the conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate and of 2,3-dihydroxymethylvalerate to α-ketomethylvalerate. The DHAD enzyme, classified by the Enzyme Commission (EC) number 4.2.1.9, is part of the naturally occurring biosynthetic pathways that produce valine, isoleucine, leucine, and pantothenic acid (vitamin B5). DHAD-catalyzed conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate is also a common step in the multiple isobutanol biosynthetic pathways that are disclosed, for example, in U.S. Pat. No. 7,851,188. Disclosed therein is engineering of recombinant microorganisms for production of isobutanol. Isobutanol is useful as a fuel additive, and the availability of biologically-produced isobutanol can reduce the demand for petrochemical fuels.